This invention relates to a method and an apparatus for inspection of a defect of an object to be inspected, or in particular to a defect inspection method and an inspection apparatus used to check for pattern defects and foreign particles in the fabrication process of a semiconductor wafer, a photomask and a printed board.
For inspection of a semiconductor chip, a method is known in which images of a semiconductor chips formed repeatedly on a wafer are acquired by scanning a surface of the wafer and images of two adjacent chips are compared with each other (die-to-die comparison) to detect a defect. On the other hand, in the case where a pattern is repeated with a short period such as in a memory cell, a method is known in which an image is displaced by an amount equal to a integer multiple of the repetition period and compared with the original one (cell-to-cell comparison) to detect a defect. A specific method of defect detection in public domain is described in JP-A-61-212708, in which two images of adjacent chips or an image displaced by an amount equal to an integer multiple of the repetition period and the image before displacement are set in position, the difference therebetween is taken and the resulting differential image is processed with a threshold value. With regard to the die-to-die comparison, a method is known as disclosed in JP-A-2-170279 in which three images are compared to specify a defective chip. In this method, the image of a chip to be inspected is compared with the images of two adjacent chips and determined as defective only in the case where comparison with the images of both right and left chips reveals a difference more than a threshold value.
In the defect inspection method for a semiconductor chip in which three images are compared to specify a defective chip by die-to-die comparison, the image of the chip to be inspected is compared with the images of immediately adjacent chips and only in the case where comparison with the images of both the right and left adjacent chips shows a difference exceeding a threshold value, a defective chip can be specified.
In the case where the signal level of the defective part of the differential image is slightly higher than the threshold value, however, the random phenomenon such as the noise component contained in the images or the stage vibration may result in a situation where comparison with the image of the right chip produces a difference exceeding the threshold value while no such difference is obtained by comparison with the image of the left chip. Such a situation poses the problem that a defective chip cannot be specified and the particular difference cannot be detected as a defect. Also, a defect may or may not be detected by the effect of the random phenomenon while at the same time posing the problem that the stability of defect detection cannot be secured.
In the inspection method by cell-to-cell comparison, on the other hand, a single actual defect produces two peaks in the differential image. Therefore, only in the case where both peaks exceed the threshold value, the peaks are determined to be a defect to avoid erroneous noise detection. In the case where the signal level of the defective part of the differential image is slightly higher than the threshold value, however, the random noise component contained in the image poses the problem that one of the peak levels fails to exceed the threshold value and no defect can be detected.